Amorphous rare earth-transition metal alloys (RE-TM) can possess strong perpendicular anisotropy and exhibit polar Kerr effect when prepared as thin films with appropriate compositions. This property makes RE-TM films good candidates for storage media in erasable optical storage system using thermomagnetic writing and magnetooptical readout. See P. Chaudhari, N. J. Cuomo, and R. J. Gambino, "Amorphous Metallic films for Magneto-Optic Applications," Appl. Phys. Lett. 22, 337 (1973). R. P. Freese, R. N. Gardner, T. A. Rinehart, D. W. Siitari, and L. H. Johnson, "An Environmentally Stable, High Performance, High Data Rate Magneto-Optic Media," Proc. Soc. Photo-Opt. Instrum. Eng. 529,6 (1985).
Reflection of light falling at oblique incidence on the boundary between two media of different refractive indices is usually described by two Fresnel amplitude reflection coefficients r.sup.p (for incident p-polarized light) and r.sup.s (for incident s-polarization). These are real numbers for nonabsorbing isotropic media. For absorbing media, there is both a magnitude and phase change in the reflected light, so that the reflection coefficients are complex. G. A. N. Connell and D. S. Bloomberg, Mott Festschrift, Plenum, New York (1985). RE-TM magnetooptical media are both absorbing and anisotropic, so their r.sup.p and r.sup.s are complex; in addition, they possess a third complex reflection coefficient r that describes optical activity in terms of the coupling of incident p- or s- polarized light into the orthogonal polarized components of the light after reflection.
A number of traditional ellipsometric methods have been used to partially characterize optical properties of film. J. Bennett, H. Bennett, "Polarization," in Handbook of Optics (McGraw-Hill, New York; 1978), and R. M. A. Azzam and N. M. Bashara Ellipsometry and Polarized Light (North-Holland, New York, 1977). Ellipsometry measured the reflection of polarized light striking a material at oblique incidence by nulling the reflected signal with an analyzer. These measurements described reflecting properties in terms of the relative phase change .DELTA.=.phi..sup.p -.phi..sup.s, and the relative amplitude attenuation, tan .gamma.=.vertline..gamma.p.vertline./.vertline..gamma.s.vertline., that are introduced by reflection from the materials. From these quantities other properties, most often surface film thickness and optical constants can be determined. Recent attempts to extend ellipsometry have used .DELTA. and tan .gamma. to obtain the off-diagonal elements of the dielectric tensor. See G. J. Sprokel, "Reflectivity, Rotation, and Ellipticity of Magnetooptic Film Structures." Appl. Opt. 23, 3983 (1984) and G. J. Sprokel, "Photoelastic Modulated Ellipsometry on Magnetooptic Multilayer Films," Appl. Opt. 25, 4017 (1986).
U.S. Pat. No. 3,155,944 discloses use of polarized light to read a magnetically recorded thin film. Polarized light reflected from the film and rotated by the Kerr effect is passed through an analyzer which is rotated for near extinction of the reflected light when the film is magnetized in one stable state. With the analyzer held fixed in this position, when the remanent state of the film is switched by applying a suitable field, the plane of polarization of the reflected light will be rotated resulting in a substantial increase in light intensity.